Natural gas is an excellent clean energy resource, of which the primary component is methane (CH4). There are abundant reserves of methane, especially with the recent discoveries of significant deposits of shale gas in continental North America and methane hydrate in the sediments of the ocean floors, which is estimated to be at least twice of the amount of carbon in all other known fossil fuel reserves.
Over the past few decades, both the production and the consumption of world natural gas have increased continuously. The proportion of natural gas in world primary energy production rose from 9.8% in 1950 to 24% at present, and was estimated to reach up to 29% in 2020. By that time, natural gas will become the energy resource in the 21st century. However, the consumption of natural gas is still not mature, e.g., the portion of natural gas used in chemical industry is low. Due to the difficulty of methane activation and the high cost of raw chemicals (olefin, aromatics etc.) caused by the fluctuating market of crude oil, the research of efficient methane conversion to value-added products is not only a scientific challenge but also an effective way to ensure a sustainable development.
There are two basic routes to produce valuable chemicals from methane, indirect and direct conversion. Currently, the most widely used method is indirect conversion. I.e., methane is first converted to syngas with various C/H ratios by either reforming or partial oxidation, and then raw chemicals and refined oil products are converted from syngas through Fischer-Tropsch synthesis, syngas to olefin, syngas to gasoline, ammonia synthesis or many other processes. However, the indirect conversion of methane is always burdened by complicated facilities, high production cost, and especially large CO2 emission. Therefore, the study of direct methane conversion to valuable chemicals has received particular attention recently.
Direct conversion of methane can be classified into three routes: oxidative coupling of methane to ethylene (OCM), selective partial oxidation of methane to methanol and formaldehyde (SOM), and methane dehydroaromatization to aromatics (MDA). Keller and Bhasin from UCC reported the first case of the direct conversion of methane in 1982 that methane oxidative coupled to ethylene at 1023 K led to 14% of methane conversion and 5% of ethylene selectivity. Although this process has been optimized with methane conversion up to 20-40%, ethylene selectivity up to 50-80%, and ethylene yield of 14-25%, the scale-up application still suffer from many disadvantages such as high temperature oxidative condition, over oxidation of methane to CO2, separation of products, etc. The SOM process encounters similar difficulties: methanol and formaldehyde tend to further oxidation and leads to low selectivity.
In 1993, researchers from Dalian Institute of Chemical Physics (DICP) reported methane dehydroaromatization (MDA) for the formation of aromatics (mainly C6H6) and H2 at 973 K under non-oxidizing conditions in a flow reactor, using a zeolite catalyst (HZSM-5) modified with molybdenum, with the result of 6% of methane conversion and over 90% aromatics selectivity (exclusive of carbon deposit). Since this landmark discovery, many researchers have worked on this process, and a plentiful amount of encouraging progresses have been made in catalyst preparation, reaction mechanism, deactivation mechanism, and so on. Nevertheless, industrial applications are restricted by the rapid carbon deposition of the catalyst.